This invention is related to the field of infra-red Schottky barrier photodetectors, in which a metallic light absorbing layer is placed next to a semiconductor layer, forming an electrically rectifying junction which can be used to detect incident light. The present invention uses this technology in, and is also related to, the field of next generation photovoltaics in which the incoming light is efficiently converted into electrical energy.
The most conventional form of a Schottky barrier photodetector is shown in FIG. 1, consisting of a thin metallic film (101) adjacent to a semiconductor layer (102), with a reflective under layer (103) and an optional anti-reflective coating (ARC) layer (100). The operation of this class of devices relies on the photo-generation of ballistic carriers in the thin metal layer by an incident light beam (104), followed by irreversible extraction into the semiconductor layer prior to the electrons thermalizing in the metal layer. Therefore, to function, this device necessarily has a metallic layer that is thinner than the electron mean free path in the metal, such that the electrons are still ballistic when they reach the metal-semiconductor interface and are irreversibly extracted from the metallic layer to the semiconductor.
The thermalisation of electrons in a metal layer can be split into two stages, which are schematically illustrated in FIG. 2, showing the energy profiles of electrons in a metal at different times (200-202) following photoexcitation. Electrons are excited (203) to energies in excess of the fermi energy Ef (205) by absorbing light, through Drude absorption, and initially create a profile (204). At this time (200) the electrons in the population (204) are ballistic and have not suffered any interaction events with either the lattice or other electrons. After a time of 0.1-1 ps (201) the electrons will have interacted with each other and the electron population in excess of the fermi energy (206) is described as hot, because the electrons have interacted with each other and so are thermalized amongst themselves, but have not interacted with the lattice and so still have a temperature in excess of the lattice temperature. The interaction with the lattice takes place on a timescale of 1-10 ps and results in an equilibrium electron distribution (202) which has the same temperature as the lattice. It is necessary to extract carriers before this time in order to have a successful photodetector or photovoltaic cell.
A schematic illustration of the operation of a conventional Schottky barrier photodetector is shown in FIG. 3. A metallic layer deposited on an n-doped semiconductor layer normally gives rise to a rectifying Schottky barrier in the conduction band (300) of the device. Illumination with light (301) warms the electron population (302) to an electron temperature (Te) which is in excess of the equilibrium lattice temperature. These hot carriers are able to irreversibly travel over the Schottky barrier (303) and generate a current in the device. This current is used to determine the intensity of light illuminating the device in a photodetector.
Early devices were based on thin films of metal which were so thin that they were partially transparent in order to ensure they were thinner than the mean free path of electrons in the metal. These devices were somewhat limited by low efficiency, and all related devices are specifically aimed at photodetection rather than photovoltaic conversion, as ballistic extraction results in the loss of low energy incident photons. This was somewhat improved in U.S. Pat. No. 4,394,571 (Jurisson, issued Jul. 19, 1983), with the introduction of a quarter wavelength cavity to enhance metallic absorption. However, the device still relied on immediate ballistic extraction of photoexcited carriers. Further improvements have focused on wave-guiding (WO 2011/112406, Patel et al., published Sep. 15, 2011), and improved absorption in metal layers has been shown through the use of resonant absorption (U.S. Pat. No. 8,536,781, Lee at al., issued Sep. 17, 2013) and plasmonic absorption (U.S. Pat. No. 5,685,919, Saito et al., issued Nov. 11, 1997).
In addition to prior art in infra-red photodetection, the present invention may be compared to hot carrier photovoltaic cells. Hot carrier photovoltaic cells operate by extracting carriers after they have thermalized among themselves, but before they have thermalized with the lattice (i.e. at stage (201) in FIG. 2). These devices have been the subject of investigation since then 1980s and one based on U.S. Pat. No. 8,975,618 (Dimmock et al., issued Mar. 10, 2015) has recently been realised. However, all such cells rely on absorption in a thin semiconductor layer and thus are limited in efficiency due to low total light absorption.